Google Ads is introducing a uniqueness check for Lookalike user lists in the Google Ads API to improve system stability and ensure fair resource allocation.
The change will be enforced starting April 30, 2026, and mainly affects Demand Gen campaigns.
Under the new rules, a Lookalike list will be treated as a duplicate if it matches an existing list across all three attributes:
– Seed lists (source audiences)
– Expansion level (similarity range)
– Country targeting (geographic scope)
If advertisers attempt to create a duplicate Lookalike list after the enforcement date:
– API v24 and later will return the error DUPLICATE_LOOKALIKE
– Earlier API versions will return RESOURCE_ALREADY_EXISTS
What advertisers and developers should do:
Reuse existing audiences instead of creating identical Lookalike lists.
Update API error handling logic to properly manage the new duplicate-list errors and ensure integrations continue to run smoothly.
Most advertisers are expected to see minimal impact, as current usage patterns generally comply with the new constraints.
